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Pregnant women are being warned not to travel to the Olympics in Brazil after a virus causing thousands of babies to be born with unusually small heads swept through the region.

Researchers have linked the rise in Microcephaly, a neurological disorder in which infants are born with smaller craniums and brains, to the Zika virus, a mosquito-borne disease first seen in Africa in 1947.

The outbreak has spread with such speed that the US-based Centers for Disease Control and Prevention has now warned pregnant women not to travel to Brazil and 13 other countries in Latin America and the Caribbean - potentially affecting thousands intending to arrive in Rio for the Olympics this August.

Meanwhile, women in Brazil and Colombia have been warned to take precautions against mosquito bites, and other women not to get pregnant until the effect of Zika on unborn children is better understood.

But Marcos Espinal, head of the Pan American Health Organization's communicable diseases department, said 'travel restrictions will not stop the spread of Zika' and it is likely to reach throughout Latin America.

'It's a mosquito that is endemic in the whole of Latin America and the Caribbean, and the population of the Americas was not exposed to the virus, so there's no immunity to it,' he said.

A female Aedes aegypti mosquito acquires a blood meal on the arm of a researcher at the Biomedical Sciences Institute in the Sao Paulo's University, Brazil. Photo / AP

So far, Brazil has recorded 3,893 cases of microcephaly, with cases in every single state of the South American country.

The condition leads to irreversible neurological damage that affects movement and vision.The number of reported deaths of deformed babies rose to 49, ministry officials said at a news conference earlier this week.

But so far, health authorities have only confirmed six cases of microcephaly where the infant was infected with the mosquito-born Zika virus.

Colombia has the second highest infection rate after Brazil, with more than 13,500 people infected with the Zika virus and the disease could hit as many as 700,000, its health minister said.

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Although microcephaly has not been definitively connected with Zika, experts believe there is a link, as the virus has been found in brain tissue and amniotic fluid from babies who were born with microcephaly or died in the womb, Brazil's Health Ministry said.

In Colombia, there are 560 known cases of pregnant women who are infected with the Zika virus, and who are being closely monitored by health workers. So far no cases of newborns suffering from microcephaly have been recorded.

But on Tuesday, Brazilian researchers took another step towards proving Zika causes microcephaly.

The Fiocruz biomedical center in Curitiba announced it had found Zika in the placenta of a woman who had a miscarriage, proving the virus can reach the foetus.

Until now, researchers had only found Zika in the amniotic fluid of two pregnant women.

'This is a significant advance, but we still cannot scientifically state that Zika is the cause of microcephaly,' said Jean Peron, an immunology expert who is experimenting on pregnant mice at the University of Sao Paulo's Institute of Biomedical Sciences.

The Zika virus is transmitted by the Aedes aegypti mosquito, which is also known to carry the dengue, yellow fever and Chikungunya viruses.

Last week, U.S. health authorities confirmed the birth of a baby with microcephaly in Hawaii to a mother who had been infected with the Zika virus while visiting Brazil last year.

In Colombia, which has the second highest Zika infection rate after Brazil, the government is advising women to delay becoming pregnant for six to eight months to avoid the risk.

It is thought the Zika virus - which was at first thought to be relatively innocuous - may have arrived in Brazil during the 2014 World Cup carried by visitors from French Polynesia, where an outbreak had just occurred.

'The virus found the perfect conditions in Brazil: a very efficient vector that loves human blood, millions of susceptible victims with no antibodies, ideal climate, and lots of places to breed,' said Ricardo Lourenso, who studies tropical infectious diseases at Brazil's Oswaldo Cruz Institute.